what is Basal metabolic rate?

Need some help for exams.. So I'm reading about energy right now for Home.Ec and this it what it says Energy requirements: The body needs a basic amount of energy to stay alive - to keep vital organs such as the heart and digestive organs working. This is known as the basal metabolic rate (BMR) Extra energy is required for the activities we do, such as walking or running. the more active the body, the more energy is used. My point is I don't really understand what bmr is so if you could say all of that in the simplest way possible, I would be grateful. Thanks!